Wireless LANs (Local Area networks) differ from wired LANs in that various radio transmission channels may interfere with one another. Indeed signal propagation is further subject to many variations in signal strength due to such factors as multipathing a result of the various propagation paths a wireless signal may experience and other factors that interfere with a clean signal. Diversity is one means of dealing with these various effects.
Antenna spatial diversity is one of the most powerful techniques for improvement of radio channel quality. The technique assumes that radio signals arrive at antennas displaced in space via substantially different propagation paths when a multipath (Rayleigh) propagation environment is present. Thus, the signals arriving at each antenna are substantially decorrelated (provided large enough antenna spacing is used), and impairments which may affect each of them due to multipath fading are mostly non-overlapping in time and frequency. Using the two signals in combination, with an appropriate combining technique based on a quality metric (e.g. received signal strength), can allow better communication quality to be sustained.
In order to capitalize on the full value of antenna diversity, it is conventional to implement duplicate receivers for each antenna path. Although this “combinational” diversity approach is very effective, it can be costly and difficult to implement in a low power environment. A simpler approach, using a smaller number of receivers than the number of antennas is called “switched diversity”. In these implementations, the receiver uses one of the antennas to recover the desired signal while monitoring the quality metric. If the quality metric falls below an acceptable level, an RF switch is actuated to connect the receiver instantaneously to a different antenna.
Usually switched diversity usage is relegated to analog transmission systems (e.g. analog cellular) or digital systems, which can accommodate retransmission of unacknowledged or negatively acknowledged messages. For systems in which the radio channel remains stationary for an acceptable interval, switched diversity may be applied using a large number of antennas. Conventional switched diversity systems, however, do not cooperate with MAC protocols since the switching of the antenna is autonomous at the receiver.
Wireless LANs are now being contemplated for delivery of time-bound multimedia communications in addition to their current use for non-time-bound data. Protocols have been developed for providing scheduled, non-conflicting time intervals for transmission of multimedia packets whose latency requirements cannot accommodate conventional retransmissions for error correction. Because delays caused by ack/nak-directed retransmission cannot be tolerated, one must seek other means to reduce error rate. Forward error correction coding is usually used for such purposes, but its use may incur large coding overheads in the case of multipath propagation environments where a significant number of symbols may be eliminated by a fade at a single antenna.
Although combinational diversity is an attractive means of reducing error rate in multipath environments, wireless LAN clients frequently require low dissipation and small (PCMCIA) form factors, and are less able to support the complexity and cost of multiple receivers. Eventually, VLSI techniques will succeed in meeting the size, cost, and dissipation requirements for these clients. However, it would be advantageous to have a means by which the switched-diversity architectures in use for today's wireless LAN receivers could be utilized to provide the necessary improved BER performance in association with software-based protocol and coding techniques.